The present invention relates to a spring unit for use in a mattress or a box spring.
A conventional spring unit used in a mattress or a box spring comprises a plurality of main springs. Each main spring is made of a single wire and has a straight rod portion and two spring protions. The upper ends of these spring portions are coupled to the ends of the rod portion. The main springs are disposed on a rectangular base plate. They are divided into a first group of long main springs and a second group of short main springs. The long main springs are arranged parallel to the long sides of the base plate and the short main springs are arranged parallel to the short sides of the base plate. Hence, the main springs of both groups form a lattice. The lower ends of the spring portions are secured to the four edges of the base plate. The rod portions are reinforced by intermediate support springs. Like the main spring, each intermediate support spring has a straight rod portion and two spring portions. The upper ends of the spring portions are coupled to the ends of the rod portion, and the lower ends of the spring portions are secured to the base plate.
In the conventional spring unit, the spring portions of the main springs and intermediate support springs are torsion bar springss. A torsion bar spring deforms very slightly when a compressive load is applied on it. However, it will be permanently deformed if a compressive load is applied on it many times. The known spring unit, which comprises torsion bar springs, may lose its elasticity in a relatively short time.